The present invention relates to an article for use under high vacuum at high temperatures or, more particularly, to an article, such as a crucible for the molecular-beam epitaxy cell, made, for example, from pyrolyric boron nitride and to be used under high vacuum at high temperatures, which is ready for service.
Articles, such as a crucible for use as a molecular-beam epitaxy cell, are usually made from pyrolytic boron nitride, referred to as PBN hereinafter, and the PBN crucible is set on a source cell for the molecular beam which is mounted on a so-called K-cell of a molecular-beam crystal growing apparatus. The crucible is charged with a source material of molecular beams, such as gallium, aluminum, arsenic, silicon and the like, and heated in an atmosphere of an extremely high vacuum to generate a molecular beam by the evaporation of the source material while the molecular beam targetted at the surface of a substrate wafer of, for example, a semiconductor silicon or germanium or a compound semiconductor such as gallium arsenide, indium phosphide and the like to grow an epitaxial layer thereon. It is essential in this process of epitaxy that the atmosphere inside the molecular-beam crystal growing chamber is under an extremely high and stable vacuum of 10.sup.-10 to 10.sup.-11 Torr.
It is known that degradation in the quality or purity of the epitaxial layer formed by the molecular-beam method is caused by the gaseous emission from the PBN crucible and other articles in the chamber because the process is performed at a high temperature under an extremely high vacuum mentioned above so that even a trace volume of the gaseous emission causes a great increase and fluctuation in the vacuum pressure to seriously affect the quality of the epitaxial layer even by setting aside the problem of an increased load on the vacuum pumps to maintain the high vacuum.
Accordingly, it is an established practice that the PBN crucible before charging with the epitaxial source material is set on the K-cell and subjected to a bake-out treatment at a high temperature under a high vacuum to effect degassing. A problem in this degassing bake-out treatment is that degassing can hardly be complete unless the treatment is performed for a considerably long time. When the bake-out treatment is performed at a high temperature of 1000.degree. C. to 1500.degree. C. under a high vacuum of 10.sup.-6 to 10.sup.-9 Torr, for example, complete degassing of a PBN crucible set on the K-cell and other articles can be accomplished only after continuing the bake-out treatment for 4 days to 9 days consequently to greatly decrease the working efficiency of the instruments resulting in a great increase in the production costs of the epitaxial products.